'Probably' is another way to say most likely or almost certainly. In the example, the speaker is 95% certain that his favorite food it sushi. Notice the following:

It will probably rain tomorrow.

We can probably come to your graduation.

at least

I eat sushi at least two times a week.

To eat sushi 'at least' two times a week is to eat it two or more times a week. For sure you eat sushi two times a week, but maybe some weeks you eat it more, but never less than twice. Notice the following:

He drinks at least 3 cans of Coca-Cola every day.

I need at least 7 hours of sleep each night.

usually

My favorite drink. Well, Coca-Cola usually.

If you drink Coca-Cola most of the time, more than any other soda, then you 'usually' drink Coca-Cola. Something you usually do is your habit. Notice the following:

He usually arrives late for work.

Do you usually drink coffee or tea?

gain weight

If I drink too much Coca-Cola I gain weight.

To 'gain weight' is to increase how much you weigh, the pounds or kilos of your body. For most people this means to get fat or fatter. Notice the following:

She gained a lot of weight in her first year after university.

Is it easy for you to gain weight?

dessert

What's your favorite dessert.

'Dessert' is a sweet, like cake or ice cream, that you eat after a meal. Notice the following: